428 Note 31: Cavendish's measures of electric shocks 



Experiments by the Editor. 



Induction coil 0-000014 0-0013 0-468 



do. o-ooooii 0-0013 0- 534 



Condensers 0-00036 0-0112 0-670 



Experiments on the prepared nerve and muscle of a frog, 

 o-ooooi 0-014 0-640 



This value of p does not differ much from 0-652, the only result which 

 Cavendish has deduced in a numerical form from his experiments. 



The most unaccountable of all the results arrived at by Cavendish is one 

 which seems to have perplexed him so much that he has left the account of the 

 experiments among which it occurs in a very impeffect state. He found (Arts. 

 639, 644) that the shock of a Ley den jar taken through a long thin copper wire 

 produced a more intense sensation than when it was taken from the jar directly. 



As in some of the experiments the wire was wound on a reel, and therefore 

 the self-induction of the current might produce an oscillatory discharge, the 

 physiological effects of which might be different from those of the simple dis- 

 charge; I charged two Leyden jars to the same potential, using Thomson's 

 Portable Electrometer as a gauge electrometer, and took the discharge of one 

 through the secondary wire of an induction coil, the resistance of which was 

 about 1000 Ohms, and that of the other through an ordinary resistance coil 

 of 1000 Ohms. 



In every trial I found that the sensation was more intense when taken 

 through the ordinary resistance coil than when taken through the induction 

 coil, and it is manifest that in the latter case the current begins and ends much 

 less abruptly, so that the result is quite in accordance with the modern theory, 

 that the sensation depends on the rapidity with which the strength of the current 

 changes. I am, therefore, quite unable to account for the opposite result obtained 

 by Cavendish. At the same time it is quite impossible that Cavendish could be 

 mistaken in this comparison of the intensity of his sensations, for he had more 

 practice than any other observer in comparing them, and he repeated this 

 experiment many times. 



The only apparent objection to the experiment is that the resistance of the 

 copper wires was only 430 in one case and only 1000 in the other, whereas the 

 resistance of a man's body, from one hand to the other, varies from about 

 1000 when the hands are thoroughly wet, to about 12,000 when they are dry, 

 so that the resistance of the copper was small compared with the possible varia- 

 tions of the resistance of Cavendish's body. 



The resistances of the tubes filled with solutions of salt, &c., were very 

 much greater, being from 20,000 to 900,000. 



